1. Field of the Invention
The invention relates to wet development of negative resist patterns (images). More particularly, the invention relates to the preparation of negative resist images substantially free of snaky lines or edges. Especially the invention relates to the preparation of negative resist images substantially free of snaky lines or edges and of a thickness substantially that of the initial resist layer. Further, the invention relates to the bonding of the irradiated resist to a siliceous substrate.
2. Description of the Prior Art
The invention is directed, in the main, to wet development of irradiated negative resist to produce a pattern (image) which is substantially undistorted from the initial pattern prior to development. Specifically, the lines, edges, and profiles are substantially free from dimensional changes such as snaking or other distortions. This development process is used in conjunction with numerous negative resist polymers which afford a range of radiation sensitivity.
When electron beam images with narrow lines, especially lines less than one .mu.m in width, are developed in negative resists, there is a pronounced tendency for the lines and edges to undergo a characteristic sinusoidal deformation that can be named "snaking". Other observers have described this line phenomenon as "sinuous lines", or "scriggily lines", or "wavy lines". These snaky lines form lateral filaments, in some instances, which cause bridging of the adjacent lines; in severe cases, snaky lines may not only bridge but merge.
It has been observed that the developed negative resist is swollen and is thicker (higher) than the thickness (elevation) of the initial resist layer. This increased thickness may introduce undesirable anomalies into the developed resist; a developed resist having substantially the thickness of the initial resist layer is preferred.
The article, "Three Dimensional Behavior of Negative Electron Resists" by R. D. Heidenreich and G. W. Kammlott, Polymer Engineering and Science, June 1977, Vol. 17, No. 6, pp. 377-380, is concerned with the influence of swelling in the exposed (irradiated) resist during solvent developing. The polymer used was poly(glycidyl methacrylate-co-ethyl acrylate). The developer was a solution mixture of methyl ethyl ketone and ethanol; two proportions were used: 5MEK:2 ethanol and 5MEK:1.5 ethanol. The development was by immersion. When the 5:2 solution was immediately followed by the 5:1.5 solution, the exposed resist developed lines were very scriggily. It was concluded that the poor edge definition of the developed resist is the result of swelling of the partially cross-linked polymer in the liquid developer.
The article, "Sol-Gel Behavior and Image Formation in Poly(glycidyl methacrylate) and Its Copolymers with Ethyl Acrylate, by E. D. Feit, M. E. Wurtz and G. W. Kammlott, Journal of Vacuum Science and Technology, 15(3), May/June 1978, pp. 944-947, is concerned with the interaction of the resist and the liquid developer. Seven samples of PGMA and eight samples of P(GMA-co-EA) were tested, using a scanning electron beam. The developer was a solution mixture of ketone and alcohol (species not specified) in proportions ranging from 3:1 to 9:1. In some instances, a high ratio solution immersion was followed by a lower ratio solution immersion. Owing to the swelling of the resist during development, lines deform and assume a sinusoidal appearance. It was concluded that faithful reproduction in the resist of a feature written by the electron beam does not depend on molecular parameters of the polymer alone, but rather on factors such as solvent induced swelling, competitive wetting of the substrate and of the polymer by the solvent, and gel rupture by forced development.
The article, "PGMA as a High Resolution, High Sensitivity Negative Electron Beam Resist", by Yoshio Taniguchi et al, Japanese Journal of Applied Physics, Vol. 18, No. 6, June 1979, pp. 1143-1148, is concerned with comparing resist material and concludes that PGMA is, under appropriate conditions, an excellent resist material. The samples were electron beam exposed; the development was by immersion or spraying with a solution mixture of methyl ethyl ketone and ethanol. The developed samples were rinsed for sixty seconds in methyl isobutyl ketone. The optimum developer was 6:1-10:1 solution mixture, with an immersion time of 180 seconds (40 seconds by spraying). It was observed that some resists showed line deformaton, referred to as "rough edges", which was attributed to post-baking temperatures.
The article, "Chloromethylated Polystyrene as a Dry-Etch-Resistant Negative Resist for Submicron Technology", by Saburo Imamura, Journal of Electro-chemical Society: Solid-State Science and Technology, Vol. 126, No. 9, September 1979, pp. 1628-1630, is concerned with a new resist material, chloromethylated polystyrene (CMS). As a resist material, CMS was irradiated with X-rays and with deep UV-radiation. The exposed CMS resist was developed by dipping into n-amyl acetate solvent for thirty seconds, and then rinsed in isopropyl alcohol for sixty seconds. No mention is made of resist line or edge deformation.
The article, "Molecular Parameters and Lithographic Performance of Poly(Chloromethylstyrene)--A High Performance Negative Electron Resist", by H. S. Choong and F. J. Kahn, Journal of Vacuum Science and Technology, 19(4), November/December 1981, pp. 1121-1126, is concerned with locating a resist material that is the equivalent of the chloromethylated polystyrene of Imamura (above). It is considered that the alkylation agent of Imamura is carcinogenic. It was found that the poly(chloromethylstyrene) polymer was equal to the Imamura polymer. The tests were carried out by exposing the resist with a scanning electron beam. The exposed resist was developed by dipping into n-pentyl acetate and then rinsed in isopropyl alcohol (as was done by Imamura). It was observed that developed lines were wavy and showed some bridging, attributed to resist swelling and the proximity of the lines.
The article, "Sensitivity and Contrast of Some Proton-Beam Resists", by Robert G. Brault and Leroy J. Miller, Polymer Science and Engineering, Vol. 20, No. 16, Mid-November 1980, pp. 1064-1068, is the product of the present Applicants. Positive and negative resist polymers were tested for proton-beam sensitivity. The styrene or substituted styrene polymers tested were: polystyrene, poly(4-chlorostyrene), poly(vinyltoluene), poly(vinylbenzyl chloride), poly(4-bromostyrene), poly(isopropylstyrene) and poly(4-tert-butylstyrene). A considerable range of sensitivity with both proton beam and electron beam radiation was observed. Development was by solvent only, using chlorobenzene or toluene.
The article, "A Method of Rapidly Screening Polymers as Electron Beam Resists", by Robert G. Brault and Leroy J. Miller, Journal of Electrochemical Society, Vol. 128, No. 5, May 1981, pp. 1158-1161, is the product of the present Applicants. In discussing the polymethacrylate positive electron resists, there is mentioned the possible use of "additional rinses with weak solvents" but no details are set forth.